Forces and stresses (AQA GCSE Design and Technology): Revision Notes
Forces and stresses
What are forces and stresses?
All materials need to withstand different types of forces and stresses to work effectively in real-world applications. When we apply forces to materials, they create internal stresses that can cause the material to change shape or even break if the force is too great.
A force is essentially a push or pull that we apply to an object. We measure forces in units called newtons (represented by the symbol N). When you apply a force to any material, it creates something called stress inside that material, which might cause it to bend, stretch, or change shape - this change in shape is called deformation.
The relationship between force and stress is fundamental to understanding material behaviour. Even small forces can create significant stresses in materials, which is why engineers must carefully calculate these effects when designing structures.
The five types of forces
Understanding how different forces work is crucial for selecting the right materials for different jobs. There are five main ways that forces can act on materials:
1. Tension
Tension is a pulling force that tries to stretch a material apart. Think of a tug-of-war game - the rope experiences tension as both teams pull in opposite directions. The rope must be strong enough to resist this pulling force without snapping.
Practical Example: Tension in Everyday Life
When you pull on a rubber band, you're applying tension force. The rubber band stretches because the material is being pulled apart at the molecular level. If you pull too hard, the tension force exceeds the material's strength and it snaps.
Common examples of tension include:
- Cables supporting a bridge
- Rope being used to tow a car
- The strings on a guitar when you tune them
2. Compression
Compression is the opposite of tension - it's a pushing or squashing force that tries to crush or compress a material. Imagine sitting on a chair - your weight creates a compression force pushing down on the chair legs, which must be strong enough to support you without breaking.
Compression forces are found in:
- Building foundations supporting the weight of structures above
- Table legs holding up a heavy load
- The concrete pillars under bridges
Materials often behave very differently under tension versus compression. For example, concrete is excellent under compression but weak under tension, which is why steel reinforcement bars are added to concrete structures.
3. Bending
Bending is more complex because it actually creates both tension and compression forces at the same time. When you bend a material like a beam, the top surface gets compressed while the bottom surface gets stretched (or vice versa, depending on which way you bend it).
The important concept here is the neutral axis - this is the imaginary line running through the middle of the material where there's neither tension nor compression. Understanding this helps engineers design beams and supports that can handle bending forces effectively.
The neutral axis is critical in bending analysis because it represents the point of zero stress. Materials above the neutral axis experience compression, while materials below experience tension. This is why I-beams are shaped the way they are - most material is placed away from the neutral axis where stresses are highest.
4. Torsion
Torsion is a twisting force that acts along the length of a material. If you've ever used a screwdriver, you've applied torsion - you twist the handle to turn the screw. The screwdriver shaft experiences torsion as it transfers your twisting motion to the screw.
Examples of torsion include:
- Turning a door handle
- Using a wrench to tighten a bolt
- The drive shaft in a car transferring power from the engine to the wheels
5. Shear
Shear forces occur when two parallel forces act in opposite directions but are slightly offset from each other. Think of using scissors - the two blades apply forces that are parallel but not aligned, which creates the cutting action.
Practical Example: Understanding Shear
When you use a paper punch, you're applying shear force. The punch creates two parallel forces - one pushing down and one pushing up (the resistance from the material below). These offset parallel forces create the clean cut through the paper.
Shear forces are present when:
- Cutting materials with scissors or snips
- Bolts holding two pieces of metal together experience shear if those pieces try to slide past each other
- Wind pushing against the side of a tall building
Static vs dynamic loads
Forces can also be classified based on whether they move or stay constant:
Static loads don't move and remain constant over time. Examples include books sitting on a shelf or the weight of a roof on building walls. These are predictable and easier to design for.
Dynamic loads are unstable or moving, which makes them more challenging to handle. A person walking across a bridge creates dynamic loads because their weight shifts and bounces slightly with each step. These forces can be more dangerous because they can create vibrations and unexpected stresses.
Dynamic loads are particularly dangerous because they can cause resonance and fatigue failure in materials. This is why engineers must consider not just the maximum force, but also how forces change over time when designing structures.
Real-world applications
Understanding these different types of forces helps us choose the right materials for specific jobs:
- Bridge cables must resist enormous tension forces
- Building foundations need to handle massive compression loads
- Aircraft wings must cope with complex bending forces during flight
- Car axles experience torsion when turning corners
- Cutting tools rely on shear forces to work effectively
Engineers must consider all possible forces when designing structures and selecting materials. A material that's excellent under one type of force might be weak under another, so understanding these concepts is essential for creating safe, reliable designs.
The failure of the Tacoma Narrows Bridge in 1940 is a famous example of what happens when engineers don't properly account for dynamic forces. Wind created oscillating forces that the bridge couldn't handle, leading to its dramatic collapse.
Key Points to Remember:
- Force is measured in newtons (N) and creates stress inside materials that can cause deformation
- Tension pulls materials apart - like a rope in tug-of-war
- Compression squashes materials together - like sitting on a chair
- Bending creates both tension and compression with a neutral axis in between
- Torsion twists materials - like using a screwdriver
- Shear involves parallel forces acting in opposite directions - like scissors cutting
- Static loads stay constant while dynamic loads move or change, making them more challenging to handle